In a principal aspect, the present invention relates to interstitial-free chromium, nickel, cobalt, molybdenum, titanium, aluminum stainless martensitic steels having an excellent combination of strength, toughness, and corrosion resistance across a variety of strength levels.
Martensitic steels exhibit high strength and toughness due to the fine sub-grain structure that forms as a result of the phase transformation from austenite at high temperature to martensite at low temperature. Martensitic steels can be classified as either containing interstitial atoms such as carbon or nitrogen, or essentially interstitial-free. Non-stainless interstitial-free maraging steels have been developed since the 1960's, and usually contain about 18 wt % Ni and substitutional elements such as Co, Mo, and Ti. The Ni content in these steels contributes to a good strength-toughness combination, by (1) increasing the thermodynamic driving force for η nucleation and thereby optimally reducing the η particle size for efficient strengthening; and (2) decreasing the Ductile-to-Brittle Transition Temperature (DBTT) and improving the matrix toughness. There are two grades in non-stainless maraging steels: C-grades, such as C-200, -250, -300, and -350; and T-grades, such as T-200, -250, and -300, where the number stands for the approximate tensile strength, in units of ksi. The C-grade contains Co and achieves higher strength for equivalent η phase fraction than the T-grade, which is free of Co and contains a higher amount of Ti. The improved strengthening efficiency of C-grade can be attributed to the reduced η particle size, which is achieved by an increased thermodynamic driving force.
Alloys can generally be considered stainless when the thermodynamic activity of Cr is sufficient to produce a stable chromic oxide passive film that prevents further corrosion. Mo and W are known to further improve the pitting corrosion resistance. However, the addition of these elements reduces the martensite start temperature (Ms). To ensure a reasonable MS, a balance of alloying elements, particularly Cr, Ni, Cu, and Mo, is required. A series of existing stainless maraging steels have established examples of an acceptable balance: PH 17-7, 17-4PH, 15-5PH, PH 13-8, Custom 450, Custom 455, Custom 465, S240, Marval X12, Vasco734, and XPH12-9. The Cr, Ni, Cu, and Mo contents of these alloys are shown in Table 1 along with the precipitated strengthening phases.
TABLE 1StrengtheningAlloyCrNiCuMoOthersPhases17-4PH1744bcc-Cu15-5PH1553.5bcc-CuCustom 4501561.50.751Nbbcc-CuS24012921.51.2Al, 0.3Tibcc-Cu + B2-NiAlXPH12-91291.21.51.6AlB2-NiAl + bcc-CuPH 13-813821.1AlB2-NiAlPH 17-71771.1AlB2-NiAlVasco7341210.51.25Al,B2-NiAl + η0.4TiMarval X1212920.7Al, 0.3TiB2-NiAl + ηCustom 465121111.6TiηCustom 455128.521Ti, 0.5Nbη + bcc-Cu
From this array of alloy compositions, trade-offs between alloying elements can be noticed in maintaining a high MS to complete the martensitic transformation at room temperature. Some alloys, such as Custom 465, require an additional cryogenic treatment to complete the transformation. Stainless maraging steels that cannot be processed by vacuum melting to large-scale ingots are shown in Table 2. The MS of Nanoflex is too low and necessitates a sub-zero isothermal martensitic transformation and/or heavy cold working after quenching to complete the martensitic transformation, limiting its geometry to wire or blade with thin cross-section. Custom 475 [U.S. Pat. No. 6,630,103 (incorporated herewith)] is limited in ingot size due to solidification segregation problems.
TABLE 2AlloyCrNiCuMoOthersStrengthening PhasesCustom 4751184.59Co, 1.2AlB2-NiAl + TCPNanoflex129240.9Ti, 0.3Alη + bcc-Cu + TCP(R phase)
The alloys listed in Tables 1 and 2 can be characterized according to their strengthening phases that are precipitated during aging. The three most common and effective strengthening phases are η, B2-NiAl, and bcc-Cu. The bcc-Cu and B2-NiAl phases are both ordered-bcc phases with considerable inter-solubility, and can nucleate coherently in the bcc martensitic matrix, thereby providing fine-scale dispersion. Some solubility of Ti in B2-NiAl is expected, and at prolonged tempering times, a highly ordered Heusler phase Ni2TiAl may form.
The η-Ni3Ti phase is believed to have the smallest optimum particle size among intermetallic precipitates in steel, and therefore is most efficient for strengthening. This strengthening efficiency minimizes the debit of nickel in the matrix and thereby suppresses the DBTT. For this reason, the η phase is utilized for strengthening the non-stainless, interstitial-free martensitic C-grade and T-grade steels where high alloy Ni contents are easily obtained with high MS temperatures.
Besides the B2, bcc-Cu, and η strengthening phases, low-symmetry, Topographically Close-Packed (TCP) phases such as R, Laves, or μ may provide some strengthening response, although at the expense of alloy ductility. Precipitation of soft austenite particles may reduce the strength of the alloy. Finally, a small strengthening response may be obtained from precipitation of coherent, nano-scale bcc-Cr particles during tempering. However, the effect of nano-scale bcc-Cr precipitates on dislocation motion and therefore mechanical properties are expected to be small.
Maraging steels may also be characterized by their strength-toughness combinations. FIG. 1 illustrates the strength—toughness combinations of a variety of commercial stainless maraging alloys, together with examples of the subject invention as discussed hereinafter. The alloys strengthened by bcc-Cu generally exhibit a yield strength of 140-175 ksi. The B2 strengthened PH13-8 alloy has good corrosion resistance and can achieve a yield strength up to about 200 ksi. The PH13-8 SuperTough® alloy has been developed by Allvac to increase the toughness of the alloy by minimizing O, N, S, and P, while maintaining strength. Additional alloys have been developed to achieve yield strength up to about 240 ksi, however their impact toughness decreases dramatically above about 235 ksi. Stainless maraging steels capable of achieving a yield strength greater than about 255 ksi are Custom475 and NanoFlex, however both suffer from aforementioned processing issues.
Maraging steels may also be characterized by corrosion resistance. Pitting Resistance Equivalence Number (PREN) is a commonly used parameter to estimate corrosion resistance. While PREN does not consider the microstructural effects on specific corrosion mechanisms, it is effective when comparing similar microstructures. PREN is defined as wt % Cr+3.3*(wt % Mo+½ wt % W), and is incorporated as a design parameter in the subject invention.
The stainless maraging steels Custom465 by Carpenter Technologies and NanoFlex, also referred to as 1RK91 by Sandvik steels employ a strengthening η phase. However, NanoFlex is specified with greater than 0.5 wt % Cu in the alloy, while Custom465 has a higher Ti content and does not contain any Co.
Two other patented alloys have shown similar strength-toughness combinations. First, Custom475 includes very high Al and Mo contents. This alloy demonstrated high strength-toughness properties, however, it can only be produced in small section sizes [U.S. Pat. No. 6,630,103, column 5, lines 46-58]. Second, a patent from Allvac for PH13-8 SuperTough describes how to make the existing, non-proprietary alloy, PH13-8 with higher toughness. However, the composition of PH13-8 SuperTough has very low Ti content.
NanoFlex must be plastically deformed to complete the martensitic transformation [U.S. Pat. RE36,382 (incorporated herewith)]. NanoFlex is suitable only for small-dimension applications, and utilizes Cu primarily to achieve the desired ductility, but also to achieve the desired tempering response.
Thus, there has remained the desire for a sizable high strength, tough stainless steel alloy.